1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically, to optical amplifiers.
2. Description of the Related Art
Optical communication systems employ optical amplifiers, e.g., to compensate for signal attenuation in optical fibers. One type of amplifier that may be used in a fiber-based optical communication system is an optical fiber amplifier (OFA). Many current optical communication systems employ erbium-doped and/or Raman fiber amplifiers. These amplifiers are examples of phase-insensitive amplifiers (PIAs). More specifically, a PIA produces signal gain that is independent of the signal phase. In contrast, a phase-sensitive amplifier (PSA) produces signal gain that does depend on the signal phase. Potential advantages of using PSAs instead of or in addition to PIAs in optical communication systems include, but are not limited to, noise reduction, the reduction of noise-induced frequency and phase fluctuations, dispersion compensation, and suppression of modulational instability. A more detailed discussion of these and other potential advantages of PSA use in optical communication systems can be found, e.g., in the following publications: (1) R. Loudon, “Theory of Noise Accumulation in Linear Optical-Amplifier Chains,” Institute of Electrical and Electronic Engineers (IEEE) Journal of Quantum Electronics, vol. 21, pp. 766-773 (1985); (2) H. P. Yuen, “Reduction of Quantum Fluctuation and Suppression of the Gordon-Haus Effect with Phase-Sensitive Linear Amplifiers,” Optics Letters, vol. 17, pp. 73-75 (1992); (3) Y. Mu and C. M. Savage, “Parametric Amplifiers in Phase-Noise-Limited Optical Communications,” Journal of the Optical Society of America B, vol. 9, pp. 65-70 (1992); (4) R. D. Li, P. Kumar, W. L. Kath, and J. N. Kutz, “Combating Dispersion with Parametric Amplifiers,” IEEE Photonics Technology Letters, vol. 5, pp. 669-672 (1993); and (5) W. Imajuku and A. Takada, “Reduction of Fiber-Nonlinearity-Enhanced Amplifier Noise by Means of Phase-Sensitive Amplifiers,” Optics Letters, vol. 22, pp. 31-33 (1997), the teachings of all of which are incorporated herein by reference. However, PSAs suitable for use in fiber-based optical communication systems are not yet sufficiently developed.